The bandwidth shortage increasingly experienced by mobile carriers has motivated the exploration of the underutilized Millimeter Wave (mmWave) frequency spectrum between 3G and 300 GHz for the next generation broadband cellular communication networks. The available spectrum of mmWave band is two hundred times greater than the conventional cellular system. The mmWave wireless network uses directional communications with narrow beams and can support multi-gigabit data rate. The underutilized bandwidth of the mmWave spectrum has wavelengths ranging from 1 mm to 100 mm. The very small wavelengths of the mmWave spectrum enable large number of miniaturized antennas to be placed in a small area. Such miniaturized antenna system can produce high beamforming gains through electrically steerable arrays generating directional transmissions.
With recent advances in mmWave semiconductor circuitry, mmWave wireless system has become a promising solution for real implementation. However, the heavy reliance on directional transmissions and the vulnerability of the propagation environment present particular challenges for the mmWave network. In general, a cellular network system is designed to achieve the following goals: 1) Serve many users with widely dynamical operation conditions simultaneously; 2) Robust to the dynamics in channel variation, traffic loading and different QoS requirement; and 3) Efficient utilization of resources such as bandwidth and power. Beamforming adds to the difficulty in achieving these goals.
Analog beamforming is a good candidate for application in mmWave beamforming wireless systems. It provides array gain for compensating severe pathloss due to harsh wireless propagation environment, and removes the need for training channel response matrix between multiple antenna elements at TX/RX sides. Different beamformers can have different spatial resolution. For example, a sector antenna can have shorter by wider spatial coverage, while a beamforming antenna can have longer by narrower spatial coverage. To provide moderate array gain, large number of array elements may be needed. In principle, beam training mechanism, which includes both initial beam alignment and subsequent beam tracking, ensures that base station (BS) beam and user equipment (UE) beam are aligned for data communication.
To ensure beam alignment, beam-tracking operation should be adapted in response to channel changes. Too fast tracking causes high overhead, too slow tracking causes beam misalignment. Beam tracking operation is analogy to link adaptation operation. For proper link adaptation operation, relevant channel state information (CSI) should be collected and provided to the scheduler (e.g., the base station). However, in mmWave systems, transmission path lifetime is expected one order of magnitude shorter than traditional cellular bands due to wavelength difference. Combined with dedicated beam with small spatial coverage, the number of effective transmission paths for a dedicated beam could be rather limited, thus more vulnerable to UE movements and environmental changes. Deciding and adapting CSI reporting periodicity thus becomes important. Similarly, it is desirable to enable beam misalignment detection for properly adapting the beam tracking operation in mmWave beamforming systems.